A frequency synthesizer based on a phase-locked loop (PLL) structure is widely used in a wireless radio frequency transceiver, and is configured to generate a local oscillation signal, which is briefly referred to as a local oscillator (LO), to implement a frequency shift of a signal.
In modern wireless communications, a radio frequency transceiver is required to be capable of working in multiple frequency bands, and therefore, it is necessary that an output signal of a phase-locked loop can cover a wide enough frequency range.
To implement a wideband phase-locked loop, a core device, that is, a voltage-controlled oscillator (VCO) in the phase-locked loop also needs to work in a wide enough frequency range. As limited by noise requirements, process conditions, or the like, the voltage-controlled oscillator generally uses a multi-band design structure, that is, multiple subbands are used to cover a wide enough frequency range, but each subband only needs to cover a relatively narrow frequency. A subband is controlled in a manner of a digital code.
In an actual application, a system requires a phase-locked loop to output a specific frequency; and for a multi-band voltage-controlled oscillator, a proper subband needs to be set, so as to meet a requirement of the system on an output frequency, and also meet a requirement on stable working of a phase-locked loop circuit.
Due to fluctuation of a chip manufacturing process and uncertainty of an application environment, an output frequency of each subband of a wideband voltage-controlled oscillator is not stable or fixed. That is, it is very difficult to directly obtain a set value of a subband of a voltage-controlled oscillator according to a target frequency value that is set by the system and needs to be output by the voltage-controlled oscillator.
The following introduces a method for obtaining, by using a target frequency value provided by a system, a subband of a voltage-controlled oscillator in the prior art.
Referring to FIG. 1, FIG. 1 is a schematic diagram of a binary search method in the prior art.
In FIG. 1, a voltage-controlled oscillator that has 16 frequency bands and is controlled by a 4-bit (bit) word is used as an example; and a node in the figure is indicated by a binary control word.
In FIG. 1, up indicates an uplink search path, and do indicates a downlink search path.
A binary algorithm is used to measure, starting from an intermediate subband of the voltage-controlled oscillator, an output frequency of the voltage-controlled oscillator by using a frequency counter, compare obtained information with a target frequency value set by a system, and select a search path according to a comparison result.
For example, a search path in FIG. 1 is: 1000→1100→1110→1101. 1101 is a target subband to be searched for. 1000 is an intermediate subband among the 16 frequency bands.
For each node on the path, counting duration of the frequency counter is R×TREF, and frequency counting precision that can be obtained theoretically is fRES=fREF/R, where fREF is a reference frequency for counting, and the reference frequency is a frequency of an input reference clock of a phase-locked loop. fRES is target precision. R can be obtained by using fREF and fRES.
R is a counting time reference parameter.
Therefore, if other overheads are not counted, total calibration duration is tCAL=NCT×R×TREF. NCT is a bit width of a subband control word of a voltage-controlled oscillator. In FIG. 1, NCT=4.
A main problem of the prior art shown in FIG. 1 is that a calibration time is relatively long. The calibration time in the art is estimated in the following by using parameters commonly used in a design of a wireless radio frequency transceiver.
                    f        REF            =                        20          ⁢                                          ⁢          MHz          ⁢                                          ⁢          indicates          ⁢                                          ⁢          that          ⁢                                          ⁢                      T            REF                          =                              1                          f              REF                                =                      50            ⁢                                                  ⁢            ns                                ;                      f        RES            =              200        ⁢                                  ⁢        kHz              ;              R      =                                    f            RES                                f            RES                          =        100              ;  
a subband control word of a voltage-controlled oscillator is generally designed to be NCT=8, and on the premise of not counting other overheads, a time required by calibration is:tCAL=NCT×R×TREF=40 us; and
when other overheads are counted, a calibration time is generally near 50 us, and for a wireless radio frequency transceiver, a too long calibration time causes a time pressure on other operations of the system.
Therefore, a person skilled in the art needs to provide a phase-locked loop frequency calibration method, which can shorten a calibration time.